I have read Jacob Nielson's usability book and he says links should never open in a new window. Here are some of his reasons:

Novice users cannot manage multiple windows and become disoriented.

The back button in the new window doesn't take them back to their original window.

When people want a new window, they'll do it themselves with a keyboard/mouse shortcut.

He says the only exception is links that open in an application, such as Microsoft Word DOC and Adobe PDF. May I propose a new exception? Consider the case when the user is several minutes into a complex web application. There's a link on this application that leads to some advanced help files. So as to not wipe out all of the user's work, that help link opens in a new window. Is this a legitimate exception? Will power users get pissed if the link is forced to open in a new window?

When does a website cross the line and become a web application? Can average users distinguish the difference and understand that links will behave differently in the two?
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JoJoFeb 23 '11 at 0:46

The "average users" that I know seem to have it clear in their own heads: GMail & Facebook are applications (they even lose track that GMail and Facebook require a browser!). Does this scale to other average users? I don't know.
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BevanFeb 23 '11 at 1:29

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Here's what Nielsen said in 1997 about when to open a new window. useit.com/alertbox/9710b.html I'm sure I've seen something more recent in which he adjusted some guidelines for "Web 2.0," but can't find it now.
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Patrick McElhaneyFeb 23 '11 at 15:55

@JoJo: there's a grey area, and the user does not need to makethe distinction.
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peterchenFeb 24 '11 at 9:59

@Bevan I'm not so sure I agree that Facebook is an "application". It definitely feels like a site, to me, which complicates your thesis about apps and sites behaving differently.
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mehaaseAug 20 '12 at 14:33

I'm of the mindset that as soon as I hear the words "TOPIC is evil" I'm ready to discredit anything else the person says. Then I have to pull back and consider the context of when and where that was said to get a better understanding of what they mean. Famous people can be wrong (ever hear the quote that "Noone will ever use more than 640KB?")

The bottom line is that you have to consider the user interaction with the web site/application as a whole. Consider your options and which option will provide the best experience.

Opening a new window: Simple to understand, as long as it doesn't interrupt the flow of what the user is doing. It works for help pages, but it has a very heavy feel to the user. After all, they have a new window with full browser controls and everything. There really needs to be a good reason for doing this, for example launching the proper page in MSDN where the user is likely to see other topics that may apply and search through that.

Opening a popup: Simple to understand, and is less intrusive than a full new window. It can still interrupt the user's flow, but because it isn't a whole new window, it feels lighter. This is useful if you only have one page or a couple paragraphs of text, and you don't want the user navigating around help files. It's also useful for alerts.

Inline text/alerts: Simple to understand, and is the least intrusive. It provides little to no interruption to the user's train of thought. The alerts can easily be discarded so they no longer clutter the screen. Challenge: your help text or alert text has to be written specifically for this task so that it makes sense on the screen. Most useful for field level help, validation messages, and site notifications (similar to the Stack Exchange badge award notification).

How involved the help is that you want to link to suggests which path would be best for your users. Complex navigable help is best done in a completely separate window. A single page with no external links is best done in a popup. Field level help is best done inline and on demand.

Have you seen Lightbox the kinda popup which greys down the screen below and presents some kind of information or asks for some confirmation or presents a imtermediate task. Lightboxes are preffered over window popups these days and provide better usability.

I'm not going to open the new link in a popup. I was going to open a whole other page in a full window.
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JoJoFeb 23 '11 at 3:54

you can do a whole range of effects along these lines, like having a help panel appear from one side for example. The concept is the same, but different manifestations may make more sense than others in specific contexts.
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Peter BagnallSep 8 '12 at 18:12

Keep in mind that not losing context has become a very important consideration in interaction design. I would say that the best way to handle that is something more ajaxian, in which the "new window" is actually a an on screen element which shows and hides itself as needed. You will, in that case, need to gracefully degrade if there is any chance that your users would be missing some of the technology needed for your ajaxian solution.

The issue here is rather the "wipe out all of the user's work" part - Why would navigating to a help page and going back again do that? On Stack Overflow it certainly doesn't, and neither does it on most basic websites. The problem are those "special place reserved in Hell" web dev people that think it's a good idea to wipe a form whenever you navigate there.

Edit based on @PeterBagnall's comment: In summary, web developers should follow both recommendations - Open links in the same window, and work with modern browsers to enable users to go back and continue their work.

Um, what are you talking about - on Stack Overflow the help opens in a new window. And if you navigate somewhere else and return to a page, it most certainly DOES wipe out what you had typed previously.
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Charles BoyungFeb 23 '11 at 13:57

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@Charles, I think what l0b0 is saying is that it shouldn't. What you should do instead is have the help link submit the form contents to temporary storage, and then show the help. Then you can repopulate the form with what they'd entered when they return. Of course, if you don't take steps to ensure this then unless the browser does it for you (and some do) then you could end up with a mighty annoyed user. I'm always a little cautious about clicking help links for that reason and will often explicitly open them in a new tab to avoid this problem - but of course I shouldn't have to.
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Peter BagnallSep 8 '12 at 18:10

There is growing acceptance of help being in a new window, usually a smaller one. My preference would be for in-line help, perhaps using AJAX. In other words, if the user clicks on a help icon, a portion of the window expands to show the help. Clicking on the same icon again hides it.

Do you make the distinction between new windows and new tabs, or do you treat them the same?
On many occasions, both website owners and the users will prefer links to open new tabs.

For the website owner, it's preferable to open external links in a new window, so as not to send the user away from the website, or at least not to throw them out actively.

For the user - when they're browsing any list of items, such as a comparison/survey of websites, a news website, or basically anything with a lot of links, they will very often prefer to open a few items in new tabs, to be read once they've finished browsing the list. And if clicking a link takes them away from the list and makes them dive into the item, they'll either make the items open in new tabs manually, or will give up after only reading an item or two.

Is there a way to specify whether a link opens in a new window or new tab? I always thought that was a browser option set by the user, not programmed by the web devloper. All we can say is <a href="foo.html" target="_blank">link</a>. There is no target="_tab" attribute.
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JoJoFeb 24 '11 at 2:41

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From what I understand, there's no way to do that in HTML, but there's kind of a hack for it in javascript - if you specify the dimensions of a new window, it does open a new window regardless of the user's preferences. However, most browsers will open it in a new tab by default, so that's ok.
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Vitaly MijiritskyFeb 24 '11 at 5:46